Data transmissions in enterprise optical communication systems have not relied on overly-complicated encoding and/or decoding schemes because the technology has been more than sufficient to support desired data transmission rates. However, as computing devices become faster and the need for increased data transmission rates is realized, the physical limits of optical devices will become a limiting factor. Accordingly, optical communication systems will begin heading toward the use of more complicated encoding and decoding schemes.
Pulse-Amplitude Modulation (PAM) is a form of signal modulation where the message information is encoded in the amplitude of a series of signal pulses. It is pulse modulation scheme in which the amplitudes of a train of carrier pulses are varied according to the sample value of the message signal. Demodulation of a PAM-encoded signal is performed by detecting the amplitude level of the carrier at every symbol period.
In a PAM4-based optical link, two Non-Return-to-Zero (NRZ)-coded two-level signals are combined together in a PAM4 encoder to create a single PAM four-level signal. The advantage of this encoding scheme is that the 4-level code utilizes the same baud, or symbol rate, of either of the two NRZ codes while containing twice the information of either. This is an attractive solution when the components of the link are baud rate limited, as is often the case for very high-speed fiber links. FIG. 1A depicts eye diagrams of the individual NRZ signals and FIG. 1B depicts the combined PAM4 signal. As seen in the comparison of FIGS. 1A and 1B, the PAM4 signal is capable of carrying twice the information of two individual NRZ signals in the same baud period.